M_SIZE
bytes_available = M_SIZE(m);
bytes_available = M_SIZE(m) - M_LEADINGSPACE(m);
if (plen < M_SIZE(m))
#define G_SIZE(x) (((x) >> S_SIZE) & M_SIZE)
#define G_SIZE(x) (((x) >> S_SIZE) & M_SIZE)
((MB_START(m) + M_SIZE(m)) - ((m)->m_data + (m)->m_len))